The following gives a good resume of Marx’s approach to political economy viz Ricardo and the Ricardian school. As others have pointed out this real science of political economy is a critique of the object of political economy and the conclusions political economists draw in their studies of this object. Furthermore, one can say it is negative in the way it reduces these economic forms to social activity but positive in the way it demonstrates how this social activity appears and reproduces itself. Finally, such an approach also demonstrates the historic specificity of a social form of production held to be trans-historical by political economy:

One can see here how the real science of political economy ends by regarding the bourgeois production relations as merely historical ones, leading to higher relations in which the antagonism on which they are based is resolved. By analysing them political economy breaks down the apparently mutually independent forms in which wealth appears. This analysis (even in Ricardo) goes so far that 1) The independent, material form of wealth disappears and wealth is shown to be simply the activity of men.Everything which is not the result of human activity, of labour, is nature and, as such, is not social wealth. The phantom of the world of goods fades away and it is seen to be simply a continually disappearing and continually reproduced objectivisation of human labour. All solid material wealth is only transitory materialisation of social labour, crystallisation of the production process whose measure is time, the measure of a movement itself. 2) The manifold forms in which the various component parts of wealth are distributed amongst different sections of society lose their apparent independence. Interest is merely a part of profit, rent is merely surplus profit. Both are consequently merged in profit, which itself can be resolved in surplus value, that is, to unpaid labour. The value of the commodity itself, however, can only be reduced to labour time. The Ricardian school reaches the point where it rejects one of the forms of appropriation of this surplus value — landed property (rent) — as useless, in so far as it is pocketed by private individuals. It rejects the idea that the landowner is an agent of capitalist production. The antithesis is thus reduced to that between capitalist and wage labourer. This relationship, however, is regarded by the Ricardian political economists as given, as a natural law, on which the production process itself is based. The later economists go one step further and, like Jones, admit only the historical justification for this relationship. But from the moment that the bourgeois mode of production and the conditions of production and distribution which correspond to it are recognised as historical, the delusion of regarding them as natural laws of production vanishes and the prospect opens up of a new society, [a new] economic formation of society, to which the bourgeois mode of production is only the transition.”
MECW 33 p 346